Many conventional FeRAM devices include a horizontal ferrocapacitor structure, in which a stack of layers is formed including top and bottom electrodes sandwiching a ferroelectric layer. An alternative “vertical capacitor” structure was suggested in U.S. Pat. No. 6,300,652, the disclosure of which is incorporated herein by reference. A vertical capacitor includes a ferroelectric element sandwiched between electrodes to either side, all at substantially the same level in the FeRAM device.
The vertical capacitors are typically to be formed over a substructure. The substructure includes various electronic components buried in a matrix (e.g. of TEOS (tetraethylorthosilicate)). The substructure further includes conductive plugs connected to the electronic components, and which extend upwards through the matrix. The upper ends of the plugs terminate in TiN/Ir barrier elements, having a top surface flush with the surface of the matrix.
Conventionally, an insulating layer of Al2O3 is formed over the surface of the matrix, and a thicker layer of ferroelectric material such as PZT (PbZrTiO3) is formed over that, and then crystallised in an oxygen atmosphere. The Al2O3 layer acts as a seed layer for PZT crystallisation, and has the further function of inhibiting oxygen diffusion into the substructure during the PZT crystallisation.
Hardmask elements are then deposited over selected areas of the PZT layer, and the portions of the PZT and Al2O3 which are not protected by the hardmask elements are etched all the way through, forming openings.
The openings are then filled with conductive material such as IrO2, by depositing IrO2 over the entire structure and chemical-mechanical planarization (CMP) polishing is performed to form a flat upper surface which is partly the PZT and partly the conductive material. Then, an Al2O3 layer is formed over the surface. The elements of IrO2 constitute electrodes, while the remaining PZT elements constitute the dielectric elements of the ferrocapacitors. At least some of the electrodes may be in electrical contact with the plugs, via the barrier elements.
The vertical capacitor structure has great potential for reducing the cell size, especially if the angle between the horizontal direction and the sides of the remaining PZT elements is high.